Method for making an electroconductive joint

ABSTRACT

A method for creating an electroconductive joint in connection with conductor rails made of copper or copper alloy, in which method in between the conductor rail elements to be joined, there is applied soldering/brazing agent, whereafter at least the junction area is heated, so that a joint is created. According to the method, the employed soldering/brazing agent is a layered soldering/brazing agent foil ( 3 ) comprising surface layers ( 4, 6 ) and an intermediate layer ( 5 ) therebetween, and the junction area is thermally treated, so that a diffusion joint is created.

The present invention relates to a method according to the preamble ofthe appended claim 1.

In plants requiring high electric power, the employed power conductorsare typically conductor rails made of copper or copper alloy. Theconductor rails are long and typically include clamp joints created bymeans of fastening means, such as bolts. Usually the joints areoverlapping joints, in which case through the pieces to be joinedtogether, there is drilled at least one hole in the transversaldirection. The holes are arranged to match, and therethrough there isinserted a fastening element, such as a bolt, which is tightened bymeans of a counterpart, such as a nut, so that the joint surfaces of thepieces to be joined are pressed together. While applying a bolt joint,the conduction of the electric current depends on the mechanical contactbetween the joined pieces. An effective and safe power conduction issecured only when the mechanical joint is in order. Clamp joints aretypically oxidized in the course of time, in which case in the junctionsthere are created transfer resistances that consume an unnecessary highamount of electric power. Heavily power-consuming units for instance inthe metallurgic industry are, among others, electrolytic plants andprocess-metallurgic electric furnaces. Owing to a weakening in theelectroconductive capacity of bolt joints and to the creation oftransfer resistances, remarkable economical losses are caused because ofan increased energy consumption. Moreover, reparations of the joints,where even ten-year-old joints should be renewed, result in high workexpenses.

The object of the invention is to realize a method and a joint wherebythe drawbacks of the prior art can be avoided. Another object of theinvention is to realize a method whereby it is possible, apart frommaking new joints, also repair old joints.

The invention is characterized in what is specified in the appendedclaims.

The arrangement according to the invention has several remarkableadvantages. By applying the method according to the invention, there areachieved extremely durable and long-lasting joints with anelectroconductive capacity that remains excellent. The method accordingto the invention is fairly easily applied in connection with differentconductor rail arrangements. The joint is quickly made, and the methodcan be applied in various different installation conditions. By means ofan embodiment of the method according to the invention, existingmechanical joints that are in poor condition can be easily repaired. Byemploying layer foil as the soldering/brazing agent, there is achievedan extremely advantageous soldering/brazing agent combination, which iseasily applicable to use. When the method according to the invention isapplied in repairing old joints, the old bolt joint can be left in placeand bypassed by using a conductive element, which is for instance hardsoldered on both sides of the old junction.

When the joining materials required for a diffusion joint are used insufficiently thin and optimized layers, it is possible, even at lowworking temperatures, to obtain, by means of diffusion mechanisms, inthe final joint structure such solid phases that have a meltingtemperature even hundreds of degrees higher than the melting temperatureof the original joint material. Thus the joint is in a wayself-correcting, because it endures higher temperatures than a purejoint material would endure. The heat required by the diffusionmechanisms between the joint material and the objects to be joined canbe brought in the process by heating with a heating device, for exampleby a liquid gas burner. Other suitable heating arrangements, such asinduction heating, can also be used for heating the junction area. Onthe surface of the soldering/brazing agent foil and/or at least in on ofthe surfaces to be joined, there is applied a layer of tin (Sn) prior tomaking the joint. The feeding of tin lowers the temperature required formaking the joint. In addition, the oxidation of the surfaces to bejoined is avoided, and protective gas arrangements are not needed inconnection with the making of the joint. In order to start the phasetransition reactions and to achieve an optimal structure in the jointseam, it suffices to have a tin layer of a few micrometers as thesurface layer of the soldering/brazing agent in between the Ag+Cu centerlayer and the object to be joined. The technique according to the methodis not critical for the Ag+Cu composition, in which case evenessentially pure Ag foil can be used. The creation of the joint takesplace rapidly as a result from the molten and solid material diffusionand the successive phase transition reactions, even at relatively lowtemperatures. By applying a preferred embodiment of the method of theinvention, soldering/brazing agents resistant to very differentoxidizing conditions have been created. With respect to corrosion takingplace in the working conditions, the tin in the created mosaic gold isnot harmful, because it is not sulfatized in the same fashion as zincand copper. As for the silver that is dissolved in the phases of thejunction seam, it helps to improve the corrosion resistance of themosaic gold. By applying the method, there are obtained extremely goodjoints that maintain their electroconductive capacity and are wellsuited to the conductor rail arrangements in plants that use anextremely high electric power.

The invention is explained in more detail below, with reference to theappended drawings, where

FIG. 1 illustrates a joint according to the invention, seen incross-section prior to the thermal treatment, and

FIG. 2 illustrates another embodiment of the method according to theinvention.

In the method according to the invention, a conductor rail 1,essentially made of mainly copper, is joined to another conductor rail 2mainly made of copper. According to the method, in between the junctionsurfaces of the conductor rails, there is fed soldering/brazing agent 3with a melting temperature that is lower than the melting temperature ofthe parts 1, 2 to be joined together; at least the junction area isheated at least up to the melting temperature of part of thesoldering/brazing agent or up to the vicinity thereof, whereafter thejunction area is cooled down. By means of the method according to theinvention, there is achieved a diffusion joint. The temperature can beraised up to the described high level so that in the junction zone,there is temporarily created a molten phase. The employedsoldering/brazing agent is layered soldering/brazing agent foil 3, whichcomprises surface layers 4, 6, and an intermediate layer 5 therebetween.According to the method, the junction area is heated so that a diffusionjoint is created.

The soldering/brazing agent is easy to handle in sheets that aresuitably wide and cut to the correct length, and it can be installed inadvance at exactly the desired spot in the joint, in which caseextremely good joints are achieved throughout the whole area of thejunction surface. The soldering/brazing agent foil is placed in betweenthe pieces 1, 2 to be joined together, so that the soldering/brazingagent 3, together with the pieces to be joined, forms a layeredstructure. In the case of FIG. 1, the proportions are adjusted for thesake of clarity, so that the soldering/brazing agent 3 is illustrated asremarkably thicker than in the reality.

The intermediate layer 5 of the soldering/brazing agent is chosen fromthe following group: a mixture of silver and copper (Ag+Cu), a mixtureof aluminum and copper (Al+Cu) or a mixture of tin and copper (Sn+Cu).With respect to their melting behavior, the compounds of thesoldering/brazing agent advantageously form eutectic compositions withcopper. For instance with a silver-and-copper soldering/brazing agent,the eutectic composition includes 71% by weight silver (Ag) and 29% byweight copper (Cu). The soldering/brazing agents can also be pure silver(Ag) or aluminum (Al).

The surface layers 4, 6 of the soldering/brazing agent foil 3 comprisetin (Sn) or some other metal that is dissolved in copper and has a lowmelting temperature, whereby the temperature required for thesoldering/brazing operation can be lowered. The surface layers 4, 6 canbe formed for example by immersing the soldering/brazing agentconstituting the intermediate layer 5 of the foil in molten tin, andwhen necessary, by rolling the foil smooth after that.

Extremely high-quality joints have been achieved by using, according tothe method, a layered soldering/brazing agent foil 3, with anintermediate layer 5 comprising silver and copper (Ag+Cu) and having athickness of 50 μm, and with surface layers 4, 6 comprising tin (Sn) andhaving a thickness of for instance 5-10 μm. The thickness of thesoldering/brazing agent foil is typically 10-500 μm, advantageously20-100 μm. When tin layers are applied on the surface, the thickness ofthe intermediate layer 5 of the foil is 10-100 μm and the thicknesses ofthe surface layers 4, 6 are 1-20 μm.

When a tin layer is employed as the surface layers 4, 6, in connectionwith soldering/brazing there can also be used an intermediate layer witha copper content that is lower than in the eutectic composition. Forinstance the copper content of the Ag+Cu soldering/brazing agent canalso be 0-29% by weight. When tin layers are used, the composition isnot critical for the method.

In FIG. 2, there is illustrated another embodiment according to themethod of the invention, where beside an old bolt joint 8, there is madea new joint that has an essentially better electroconductive capacitythan the old joint. In this embodiment, there is used a conductorelement 7, the first end whereof is connected to the first part 1 of thejoint to be repaired, and the second end is connected to the second part2 of the joint to be repaired. The conductor element 7 is connected tothe first part and/or to the second part by means of a diffusion joint3. The conductor element is designed in a way that is suitable for thejoint. Typically the joint is made by means of hard soldering/brazing,by using a layered soldering/brazing foil.

EXAMPLE I

In this example, a copper object was joined with another copper objectby an Ag+Cu soldering/brazing agent comprising, in percentages byweight, 71% Ag and 29% Cu. The soldering/brazing agent was applied inthe form of foil with a thickness of 50 μm, and in addition, on the foilsurface there was made a tin layer with a thickness of the order 5-10μm. The temperature was raised up to about 600° C. The holding time wasabout 5 minutes. The joint according to the example succeededexcellently. There was achieved a compact and smooth joint, where thetin, which was originally contained as an essentially pure element,formed a mosaic gold seam with the copper.

By means of the method according to the invention, there can be joinedconductor rails with junction surfaces that are made of copper and/orcopper alloys, typically with a copper content of at least 50%.

What is claimed is:
 1. A method for creating an electroconductive jointin connection with conductor rails made of copper or copper alloy,comprising applying soldering agent in between the conductor railelements to be joined, then heating at least the junction area, so thata joint is created, wherein the employed soldering agent is a layeredsoldering agent foil comprising surface layers formed of tin (Sn) or ametal other than tin, that dissolves well in copper and has a meltingtemperature that is lower than the melting temperature of the conductorrail elements to be joined, and an intermediate layer therebetweenhaving an eutectic composition with copper, said intermediate layer ofthe soldering agent being selected from the group consisting ofessentially silver, a mixture of silver and copper (Ag+Cu), a mixture ofaluminum and copper (Al+Cu) and a mixture of tin and copper (Sn+Cu), andthermally treating the junction area so that a diffusion joint iscreated.
 2. A method according to claim 1, wherein the surface layer isformed of tin, the intermediate layer of the soldering agent is amixture of silver and copper (Ag+Cu), and the copper content of theintermediate layer is between 0% and 29% by weight, the balance beingsilver.
 3. A method according to claim 1, wherein the intermediate layerof the soldering agent comprises, in percentages by weight, about 71%silver (Ag) and about 29% copper (Cu).
 4. A method according to claim 1,wherein the intermediate layer of the soldering agent consistsessentially of silver (Ag).
 5. A method for creating anelectroconductive joint in connection with conductor rails made ofcopper or copper alloy, comprising applying soldering agent in betweenthe conductor rail elements to be joined, then heating at least thejunction area, so that a joint is created, wherein the employedsoldering agent is a layered soldering agent foil comprising surfacelayers formed of tin (Sn) or a metal other than tin, that dissolves wellin copper and has a melting temperature that is lower than the meltingtemperature of the conductor rail elements to be joined, and anintermediate layer therebetween having an eutectic composition withcopper, said intermediate layer of the soldering agent consistingessentially of aluminum (Al), and thermally treating the junction areaso that a diffusion joint is created.
 6. A method according to claim 1,wherein the soldering agent foil has a thickness of about 10-500 μm. 7.A method according to claim 6, wherein the intermediate layer of thesoldering agent has a thickness of 10-100 μm and the surface layers havea thickness of 1-20 μm.
 8. A method for creating an electroconductivejoint to be applied particularly in connection with the reparations ofconductor rail joints made of copper or copper alloy, comprisingapplying soldering agent in between the elements to be joined,thereafter heating at least the junction area, so that a joint iscreated, wherein in the method, introducing a conductor element thatbypasses the junction spot to be repaired, connecting the first end ofsaid element to the first part of the joint to be repaired, andconnecting the second end of said element to the second part of thejoint to be repaired, and connecting the conductor element to the partsby means of a diffusion joint, by applying, between the parts to bejoined, a layered soldering agent foil comprising surface layers and anintermediate layer provided therebetween.
 9. A method according to claim6, wherein the soldering agent foil has a thickness of about 20-100 μm.10. A method according to claim 5, wherein the soldering agent foil hasa thickness of about 10-500 μm.
 11. A method according to claim 10,wherein the intermediate layer of the soldering agent has a thickness of10-100 μm and the surface layers have a thickness of 1-20 μm.
 12. Amethod according to claim 10, wherein the soldering agent foil has athickness of about 20-100 μm.